When I'm cutting the sides of a bob I pull down my first side section and comb it behind the ear to make my first cut even with the back. It will make it longer in the front (great for inverted bobs!!) so I then just even it out. Before I pull my second section down I go to the other side and do the same thing. Making sure each section matches each other. I never cut all of one side at once when doing a bob.

if you begin in the back, and follow your guide as you go around to the sides,make sure you hold your comb at the same angle on both sides. the reason this can happen is losing your guide, and tilting the comb, or overdirecting the hair. bring the hair straight down, no elevation, natural fall.try to use the same amount of tension throughout the cut.for a bob that should be zero.

start at the nape. with each section you pull down and cut - make sure this line stays perfect. check the corners for evenness each time before pulling the next section down. before starting on the sides, do a final check, parting the hair vertically just behind the ears - where the hairline changes directions. it must be parted at precisely the same place on each side. now comb the hair on the back sections straight down and check those corners one more time. when it's perfect, move on to the sides and DON'T TOUCH THE LENGTH OF THOSE CORNERS - they are your guide - and your sides should end up perfect, or at least needing minimal adjustment. also - it was mentioned before, but can't be emphasized enough - compensate for the hair over the ears bouncing up so you don't get a hole in the perimeter. there are many methods to do this, everyone has their preference. i like to tap the hair above the ear to create some slack before cutting.